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2. Where and in what forms is water available on Earth?

  • 2.1 How does water move from the atmosphere to the ground and back?
    • 2.1.1 Precipitation
    • 2.1.2 Evaporation and transpiration
  • 2.2 How much freshwater is found at the Earth’s surface?
  • 2.3 How much freshwater can be found underground?

The source document for this Digest states:

The world’s water exists naturally in different forms and locations: in the air, on the surface, below the ground and in the oceans (Figure 4.1).

Although a large volume of freshwater exists ‘in storage’, it is more important to evaluate the renewable annual water flows, taking into account where and how they move through/ the hydrological cycle (Figure 4.2).

This schematic of the hydrological cycle illustrates how elements can be grouped as part of a conceptual model that has emerged from the new discipline of ecohydrology, which stresses the important relationships and pathways shared among hydrological and ecological systems (Zalewski et al., 1997). This conceptual model takes into consideration the detail of the fluxes of all waters and their pathways while differentiating between two components: ‘blue water’ and ‘green water’. Blue waters are directly associated with aquatic ecosystems and flow in surface water bodies and aquifers. Green water is what supplies terrestrial ecosystems and rain-fed crops from the soil moisture zone, and it is green water that evaporates from plants and water surfaces into atmosphere as water vapour. This concept was developed by Falkenmark and Rockström (2004) who contend that the introduction of the concepts of ‘green water’and ‘blue water’, to the extent that they simplify the discussion for non-technical policy-makers and planners, may help to focus attention and resources on the often neglected areas of rain-fed agriculture, grazing, grassland, forest and wetland areas of terrestrial ecosystems and landscape management.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 1. Global Hydrology and Water Resources, 1b. Water’s global occurrence and distribution, p.122-123
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

Part 2. Nature, Variability and Availability

The Earth’s hydrological cycle is the global mechanism that transfers water from the oceans to the surface and from the surface, or subsurface environments, and plants to the atmosphere that surrounds our planet. The principal natural component processes of the hydrological cycle are: precipitation, infiltration, runoff, evaporation and transpiration. Human activities (settlements, industry, and agricultural developments) can disturb the components of the natural cycle through land use diversions and the use, reuse and discharge of wastes into the natural surface water and groundwater pathways.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, p.123
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.1 How does water move from the atmosphere to the ground and back?

    • 2.1.1 Precipitation
    • 2.1.2 Evaporation and transpiration

2.1.1 Precipitation

The source document for this Digest states:

The Earth’s atmosphere contains approximately 13,000 km3 of water. This represents 10 percent of the world’s freshwater resources not found in groundwater, icecaps or permafrost (Figure 4.1). This is similar to the volumes found in soil moisture and wetlands. However, of more importance is the fact that this vapour cycles in the atmosphere in a ‘global dynamic envelope’, which has a substantive annually recurring volume, estimated to be from 113,500 to 120,000 km3 (Shiklomanov and Rodda, 2003; FAO-AQUASTAT, 2003). Precipitation occurs as rain, snow, sleet, hail, frost or dew. These large volumes illustrate precipitation’s key role in renewing our natural water resources, particularly those used to supply natural ecosystems and rainfed crops. About 40 percent of the precipitation that falls on land comes from ocean-derived vapour. The remaining 60 percent comes from land-based sources. It is particularly pertinent to recognize that snowfall can contribute a large percentage of a region’s total precipitation in temperate and cold climate regions. For example, in the western US, Canada and Europe, 40 to 75 percent of regional precipitation can occur as snow.

The International Panel on Climate Change (IPCC) has published the international reference for each country’s average annual precipitation, based on the period of record from 1961 to 1990 (New et al., 1999; Mitchell et al., 2002). Countries’ precipitation ranges from 100 mm/yr in arid, desert-like climates to over 3,400 mm/yr in tropical and highly mountainous terrains. Together with temperature, they define the significant variables in global climatic and ecosystem biodiversity settings. This long-term record base determines averages and defines predictable variability both in time (monthly, annually, seasonally) and place (nations, monitoring locations). This record is significant as its 30 -year standard is commonly compared with actual annual amounts to define the relative current variability, frequently tied to regional and global evaluations of drought and climate change.

It is essential to water resources development to understand the pathways of water as it arrives in the form of precipitation and migrates through the cycle components. Table 4.1 illustrates how precipitation, in three relatively diverse climatic zones, generally either returns by evaporation or evapotranspiration back into the atmosphere, becomes surface water through runoff, or recharges groundwater.

Table 4.1 Precipitation distribution into surface water and groundwater components (by climate region)

Mapping precipitation’s isotopic composition (3H, 18O and 2H) can help trace water movement through the water cycle components. This is routinely done as part of the Global Network of Isotopes in Precipitation (GNIP)1 operated jointly by IAEA and WMO at 153 stations in 53 nations. IAEA has initiated several projects to study and distinguish among moisture sources and to better understand the cycle transport patterns using applied isotope techniques. Particular case studies have been carried out in India (Bhattacharya et al., 2003), Southeast Asia (Aggarwal et al., 2004) and with twenty-one research groups participating globally to monitor many other major rivers (Figure 4.3). This approach is of further significance as it assists in the evaluation of the hydrological cycle’s response to climatic fluctuations and can be used to calibrate and validate atmospheric circulation models used in climate change studies.

[1. See isohis.iaea.org  for more information.]

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2a. Precipitation, p.123
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.1.2 Evaporation and transpiration

The source document for this Digest states:

The processes of evaporation and transpiration (evapotranspiration) are closely linked to the water found in soil moisture; these processes act as driving forces on water transferred in the hydrological cycle. Movement through soil and vegetation is large and accounts for 62 percent of annual globally renewable freshwater. Evapotranspiration rates depend on many locally specific parameters and variables that are difficult to measure and require demanding analyses in order to calculate an acceptable level of accuracy. Other hydrological, cycle-related and meteorological data are also considered in the estimation of the rates. Today, however, local water management in basins or sub-basins can better calculate transpiration rates.

Evaporation from surface water bodies such as lakes, rivers, wetlands and reservoirs is also an important component of the hydrological cycle and integral to basin development and regional water management. In the case of artificially-created reservoirs, it has been estimated by Rekacewicz (2002) that the global volumes evaporating since the end of the 1960s have exceeded the volume consumed to meet both domestic and industrial needs.

From the standpoint of food production and ecosystem maintenance, soil moisture is the most important parameter to net primary productivity (NPP) and to the structure, composition and density of vegetation patterns (WMO, 2004). Near-surface soil moisture content strongly influences whether precipitation and irrigation waters either run off to surface water bodies or infiltrate into the soil column. Regionally, mapping soil moisture deficit is becoming a widely used technique to link climatological and hydrological information in agriculture (e.g. Illinois, US) and to reflect drought conditions (US Drought Mitigation Center, 2004). Soil moisture distribution is now identified as a prerequisite for effective river-flow forecasting, irrigation system maintenance, and soil conservation (Haider et al., 2004). Its distribution in time and place are now viewed as essential to hydrological, ecological and climatic models – both at the regional and global level (US NRC, 2000).

The Global Soil Moisture Data Bank (Robock and Vinnikov, 2005; Robock et al, 2000) archives contain data sets of national soil moisture records but the data sets are incomplete in terms of global coverage.

Satellite data can provide broader coverage with current results that can be more closely representative when combined with ground validation. From 2002, NASA’s climate-monitoring ‘Aqua’ satellite has daily records of 50 to 60 km resolution data, readily obtained from NOAA (Njoku, 2004; Njoku et al., 2004). From 2010, the ‘Hydros’ satellite will exclusively monitor daily soil moisture changes around the globe with an improved spatial resolution of 3 to 10 km (Entekhabi et al., 2004; Jackson, 2004). This will be an important upgrade for remotely-sensed soil moisture data, which are becoming increasingly relied upon by agricultural marketing and administrative boards, commodity brokers, large-scale farms, flood- and drought-monitoring and forecasting agencies, water resources planning and soil conservation authorities and hydroelectric utility companies.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2b. Evaportranspiration and soil moisture, p.124
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.2 How much freshwater is found at the Earth’s surface?

2.2.1 Snow and ice

The source document for this Digest states:

About three-quarters of the world’s entire natural freshwater is contained within ice sheets and glaciers. However, most (97 percent) is not considered as a water resource as it is inaccessible, located in the Antarctic, Arctic and Greenland ice sheets. However, land-based glaciers and permanent snow and ice – found on all continents except Australia – cover approximately 680,000 km2 and are critical to many nations’ water resources. Even in situations where ice covers only a small percent of a basin’s upland mountainous terrain (e.g. in the Himalayas, Rockies, Urals, Alps, Andes), glaciers can supply water resources to distant lowland regions. Thus, glacial ice and snow represents a highly valuable natural water reservoir. Typically it affects stream-flow quantity in terms of time and volume since glaciers temporarily store water as snow and ice and release runoff on many different time scales (Jansson et al., 2003; Hock et al., 2005). Glacial runoff characteristically varies with daily flow cycles that are melt-induced and seasonal since concentrated annual runoff occurs in summer when the water stored as snow in winter is released as stream flow. The seasonal runoff benefits occur principally in nations in the mid- and high latitudes where there are otherwise only periods of low flow, but benefits also occur in many semi-arid regions. Glaciers can also affect long-term annual water availability since runoff either increases or decreases as their mass balance decreases or increases, respectively. Finally, glaciers tend to act as stream-flow regulators that can minimize year-to-year variability when catchment areas are moderately (10 to 40 percent) glaciated. Runoff variability rises as glaciated percentage both increases and decreases. Glacier conditions are now monitored globally since climate change is affecting their size and mass balance.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2c. Snow and Ice, p.125
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.2.2 Lakes

The source document for this Digest states:

Surface waters include the lakes (as well as ponds), reservoirs, rivers and streams and wetlands our societies have depended upon and benefited from throughout history. The flow into and through these surface water bodies comes from rainfall, runoff from melting snow and ice and as base-flow from groundwater systems. While surface waters volumetrically hold only a small volume (0.3 percent) of the Earth’s total freshwater resources, they represent about 80 percent of the annually renewable surface and groundwater. Ecosystem services from surface waters are widespread and diverse as well as being of critical importance. Reservoirs and large lakes effectively counteract high seasonal variability in runoff by providing longer-term storage. Other services supported by surface waters include shipping and transport, irrigation, recreation, fishing, drinking water and hydropower.

Lakes

Meybeck (1995), Shiklomanov and Rodda (2003) and most recently Lehner and Döll (2004) have provided extensive data characterizing the world’s lakes on a global scale. Lakes store the largest volume of fresh surface waters (90,000 km3) – over forty times more than is found in rivers or streams and about seven times more than is found in wetland areas. Together with reservoirs, they are estimated to cover a total area of about 2.7 million km2, which represents 2 percent of the land’s surface (excluding polar regions) (Lehner and Döll, 2004). Most lakes are small. The world’s 145 largest lakes are estimated to contain over 95 percent of all lake freshwater. Lake Baikal (Russia) is the world’s largest, deepest and oldest lake and it alone contains 27 percent of the freshwater contained in all the world’s lakes. Lake waters serve commerce, fishing, recreation, and transport and supply water for much of the world’s population. However, detailed hydrological studies have been conducted on only 60 percent of the world’s largest lakes (Shiklomanov and Rodda, 2003). LakeNet2 is one example of an organization working with local and regional governments, NGOs and IGOs in over 100 countries in order to address this knowledge deficit, to tackle degrading conditions, and to develop lake basin management programmes that include important protection strategies. Recently, a global database of lakes, reservoirs and wetlands (GLWD) has been created and validated at the Center for Environmental Systems Research, University of Kassel (CESR, Germany) in cooperation with the World Wildlife Fund (WWF) ( Lehner and Döll, 2004). The primarily digital map-based approach, complete with fully downloadable data, facilitates the linking of existing local and regional registers and remotely sensed data with the new inventory. As such, it is an important achievement related to global hydrological and climatological models.

[2 See www.worldlakes.org  for more information.]

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2d. Surface Waters, p.125
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.2.3 Rivers and streams

The source document for this Digest states:

An estimated 263 international river basins have drainage areas that cover about 45 percent (231 million km2) of the Earth’s land surface (excluding polar regions) (Wolf et al., 1999, 2002). The world’s twenty largest river basins have catchment areas ranging from 1 to 6 million km2 and are found on all continents. The total volume of water stored in rivers and streams is estimated at about 2,120 km3. The Amazon carries 15 percent of all the water returning to the world’s oceans, while the Congo-Zaire basin carries 33 percent of the river flow in Africa (Shiklomanov and Rodda, 2003)3

Variability in runoff is depicted by river/stream flow vis-à-vis time graphs (hydrographs). In terms of variability, Figure 4.4 (Digout, 2002) illustrates the three low and three high runoff periods that were experienced in the twentieth century by documenting the natural fluctuations in river runoff in terms of both time and place. These types of periodic variations are not particularly predictable as they occur with irregular frequency and duration. In contrast, we are commonly able to predict runoff variability on an annual and seasonal basis from long-term measurement records in many river locations. River-flow graphs representative of the principal climatic regions are illustrated in Figure 4.5 (Stahl and Hisdal, 2004). Shown together with monthly precipitation and evaporation, they portray the annual variability that is relatively predictable and similar according to principal climatic regions of the world. From this climatic zone perspective, tropical regions typically exhibit greater river runoff volumes while arid and semi-arid regions, which make up an estimated 40 percent of the world’s land area, have only 2 percent of the total runoff volume (Gleick, 1993).

Monitoring networks for river flow and water levels in rivers, reservoirs and lakes, supplemented by estimates for regions where there is no extensive monitoring, help understand runoff and evaluate how to predict its variability. Measurement networks are relatively common in many developed populated areas. Most of the world’s major contributing drainage areas have relatively adequate monitoring networks in place. The Global Runoff Data Center (GRDC, Koblenz, Germany), under WMO’s auspices, routinely acquires, stores, freely distributes and reports on river discharge data from a network of 7,222 stations, about 4,750 of which have daily and 5,580 of which have monthly data (GRDC, 2005; Map 4.1). Other international programmes such as the European Water Archive (Rees and Demuth, 2000) and national data centres supplement this (data from private institutions are not included). The longer the flow record, the better we can predict variability in runoff – input that is especially important in the context of flood forecasting, hydropower generation and climate change studies. The quality and adequacy of data records for runoff vary tremendously. While some records extend back 200 years in Europe and 100 to 150 years on other continents, in many developing nations the data record is generally of insufficient length and quality to carry out either reliable water resources assessments or cost-effective project designs. As a result, for these regions, data is rarely compiled or distributed effectively on a global scale (WMO, 2005).

[3 Statistics related to the world’s river systems (length, basin area, discharge, principal tributaries and cities served) are currently updated online at www.rev.net/~aloe/river/ , as part of an open source physical sciences information gateway (PSIGate).]

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2d. Surface Waters, p.126
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.2.4 Wetlands

The source document for this Digest states:

Wetlands are water-saturated environments and are commonly characterized as swamps, bogs, marshes, mires and lagoons. Wetlands cover an area about four times greater than the world’s lakes. However, they contain only 10 percent of the water found in lakes and other surface waters. During the last century, an extensive number of wetlands were destroyed or converted to other forms of land-use. The role they play in terms of ecosystems and water services are more fully described in Chapter 5. However, because they total about 6 percent of the Earth’s land surface (OECD, 1996), they are critical areas to consider and protect in terms of surface water and, in some regions, groundwater resources. Currently, extensive work is being done through the ‘Wise Use’ campaigns sponsored principally by Ramsar, WWF and UNEP. These campaigns seek to maintain critical services in water and related livelihood and food production areas. An important new study on variability in the role of wetlands was carried out by Bullock and Acreman (2003), wherein they assess the differences in wetland water quantity functions based on 169 worldwide studies conducted from 1930 to 2002. They believe this new review ‘provides the first step towards a more scientifically defensible functional assessment system (of wetlands)’ and establishes ‘a benchmark for the aggregated knowledge of wetland influences upon downstream river flows and aquifers’. They conclude that ‘there is only limited support to the generalized model of flood control, recharge promotion and flow maintenance portrayed throughout the 1990s as one component of the basis of wetland policy formulation’, noting that support is confined largely to floodplain wetlands. They also note that: ‘Less recognized are the many examples where wetlands increase floods, act as a barrier to recharge or reduce low flows’ and that ‘generalized and simplified statements of wetland function are discouraged because they demonstrably have little practical value’. Overall they conclude that wetlands cannot be considered to have the same role in every hydrological setting. They recommend that future water management actions for both basins and aquifers carefully evaluate each wetland’s characteristics as they will exhibit different performance and functional roles according to their location in the watershed, their climate, and the extent of other development features.

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2e. Wetlands, p.127
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf

2.3 How much freshwater can be found underground?

The source document for this Digest states:

Global groundwater volume stored beneath the Earth’s surface represents 96 percent of the Earth’s unfrozen freshwater (Shiklomanov and Rodda, 2003). Groundwater provides useful functions and services to humans and the environment. It feeds springs and streams, supports wetlands, maintains land surface stability in areas of unstable ground, and acts as an overall critical water resource serving our water needs.

UNESCO and WMO support the International Groundwater Resources Assessment Centre (IGRAC, hosted in Utrecht, The Netherlands). IGRAC estimates that about 60 percent of withdrawn groundwater is used to support agriculture in arid and semi-arid climates. Morris et al. (2003) report that groundwater systems globally provide 25 to 40 percent of the world’s drinking water. Today, half the world’s megacities and hundreds of other major cities on all continents rely upon or make significant use of groundwater. Small towns and rural communities particularly rely on it for domestic supplies. Even where groundwater provides lower percentages of total water used, it still can serve local areas with relatively low-cost good-quality water where no other accessible supply exists. Finally, groundwater can bridge water supply gaps during long dry seasons and during droughts.

Occurrence and renewability

Recent, globally focused groundwater publications (Zekster and Everett, UNESCO Groundwater Series, 2004; UNEP, 2003), point out that large variations in groundwater exist in terms of occurrence, rate of renewal and volumes stored in different types of aquifers. Geological characteristics are also an important factor. While shallow basement aquifers contain limited storage, large volumes of groundwater are stored in thick sedimentary basins. Aquifers in folded mountain zones tend to be fragmented, while volcanic rock environments have unique hydraulic conditions. Shallow aquifer systems have near-surface water tables that are strongly linked to and interchange with surface water bodies. Map 4.2 illustrates the thirty-six Global Groundwater Regions identified by IGRAC (2004), which compares predominant hydrogeological environments found around the world. The UNESCO-led World-wide Hydrogeological Mapping and Assessment Programme (WHYMAP) also contributes to mapping aquifer systems, collecting and disseminating information related to groundwater at a global scale (see Chapter 13).

Groundwater, as a potential resource, can be characterized by two main variables: its rate of renewal and its volume in storage. Much of groundwater is derived from recharge events that occurred during past climatic conditions and is referred to as ‘non-renewable groundwater’ (IAEA). The actual recharge of these aquifer systems is negligible. The world’s largest non-renewable groundwater systems (Table 4.2) are located in arid locations of Northern Africa, the Arabian Peninsula and Australia, as well as under permafrost in Western Siberia. Their exploitation will result in a reduction in stored volumes. A debate has arisen about how and when to use these groundwater resources as sustainable groundwater development is understood as ‘exploitation under conditions of dynamic equilibrium leaving reserves undiminished’. However, nations may decide that the exploitation of such reserves is justified where undesired side-effects would not be produced (Abderrahman, 2003). UNESCO and the World Bank have jointly prepared the publication Non-renewable groundwater resources, a guidebook on socially-sustainable management for policy makers (forthcoming, 2006).

Transboundary groundwater

In terms of shared water resources, groundwater does not respect administrative boundaries. Most of the large non-renewable reserves in Table 4.2 are shared. However, in addition to these aquifer systems, there are numerous smaller renewable transboundary aquifers located worldwide. Attention to shared groundwater resources management is increasing with strong support from several international organizations that are addressing sustainable management strategies which would enable shared socio-economic development of such aquifers. At present, the UNESCO Internationally Shared Aquifer Resources Management (ISARM) project is compiling an inventory of transboundary aquifers.

Natural groundwater quality

Most renewable groundwater is of a high quality, is adequate for domestic use, irrigation and other uses, and does not require treatment. However, it should be noted that uncontrolled development of groundwater resources, without analysis of the chemical and biological content, is an unacceptable practice that can (as in the example of fluoride and arsenic problems in Southeast Asia) lead to serious health problems. Some waters have beneficial uses owing to naturally high temperatures and levels of minerals and gas. This is the case for thermal waters where these properties have been created by high geothermal gradients, volcanic settings or natural radioactive decay. In most cases, these groundwaters are highly developed and used for health and recreation (spa) and geothermal energy services.

Groundwater monitoring networks

Groundwater monitoring networks, as with surface water systems, operate differently at national, regional and local levels. Groundwater levels constitute the most observed parameter, whereas widespread and continuous water quality and natural groundwater discharge and abstraction networks are operational in only a few countries (Jousma and Roelofson, 2003). Several large-scale efforts are underway to upgrade monitoring and networks, for example, in Europe (Proposal for new Directive on Groundwater Protection [EC 2003] and in India [World Bank, 2005]). However, groundwater assessment, monitoring and data management activities are for the most part minimal or ineffective in many developing countries and are being downsized and reduced in many developed counties (see Chapter 13). Lack of data and institutional capacity is endemic, making adequate groundwater development and management difficult. GEMS/Water (a UNEP programme) is currently adding national groundwater data to its international water quality database (described in Part 3). This will supplement the current global knowledge of groundwater quality information collected and displayed by IGRAC on its website, which includes special reports on both arsenic and fluorides in groundwater (IGRAC, 2005a, 2005b).

Source & ©: UNESCO, The United Nations World Water Development Report 2 (2006)
Section 2: Changing Natural Systems,
Chapter 4 (UNESCO & WMO, with IAEA),
Part 2. Nature, Variability and Availability, 2f. Groundwater, p.128-130
 www.unesco.org/water/wwap/wwdr2/pdf/wwdr2_ch_4.pdf


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